The present invention relates to cryogenic refrigeration systems, and in particular to cryogenic refrigeration systems for superconducting magnetic energy storage systems and other pulsed cryogenic load systems.
Among the emerging industrial uses for superconducting components are superconducting magnetic energy storage (SMES) systems which may be used to store and instantaneously provide electrical power to offset damaging voltage dips and sags caused by various conditions, such as routine circuit-switching at power substations or peak demands at locations remote from substations.
Although lasting a fraction of a second, voltage dips can cause significant damage to electronic controllers essential to manufacturing operations. Uninterrupted power sources are thus critical to prevent idling entire manufacturing plants, and local SMES systems have been proposed and installed to protect manufacturing operations. Numerous other applications of SMES systems are possible to prevent damaging and undesirable voltage dips or voltage sags.
Voltage sags can be equally damaging to other systems. For example, in the operation of the Bay Area Rapid Transit District (BART) system, the long distance between the traction rectifier substations at each end of a 3.5 mile long trans-bay tube permits the train voltage to drop to undesirably low levels under certain conditions. Such conditions occur during peak demand times when the number of trains causes the entire power grid to sag. When trains pass near the central portion of the trans-bay tube, the voltage can dip to an undesirably low level. Train drive choppers cut out if the voltage falls below 750 volts (for a nominal 1000 volt DC system), resulting in jerky motion of the passing trains, increased train maintenance, and passenger discomfort and anxiety. SMES systems have been proposed for installation in the gallery of the BART trans-bay tunnel generally between its ends to provide needed power to prevent such sags. The frequency at which the magnet of the SMES unit will be required to pulse will be generally predictable, and presents an intermittent, non-continuous load requirement on a 24-hour cycle. By contrast, less demanding requirements are anticipated for SMES installations at manufacturing facilities, where occasional, random demand for pulsing of the magnet arises from voltage dips or sags. Equalized, uninterrupted power supplies are, thus, of importance not only to private manufacturing systems, but are necessary for safe and economical operation of major systems serving the public.
The need to equalize power supplies and prevent voltage dips provides the opportunity for SMES systems to supplement power grids and even eliminate power substations within electrical power systems. Essential to commercial viability of SMES applications is a reliable and economical cryogenic refrigeration/liquefaction system which can supply needed refrigeration at liquid helium temperatures to sustain the superconducting devices therein. Because an estimated 80% of SMES system operating costs relate to the power demands of refrigeration, highly efficient refrigeration/liquefaction systems are desired. In addition, the cryogenic temperatures at which SMES systems must operate place a premium on system design for low thermal losses and high thermal efficiency to further reduce loads which increase power demand. Finally, for some applications, the need for a compact SMES design is at a premium, such as the use of a SMES in the dimensionally restricted gallery of the BART transbay tunnel.
Accordingly, the need exists for high reliability, high efficiency refrigeration/liquefaction systems which operate with low thermal losses, to satisfy the demands of various existing and emerging applications for SMES systems and devices, and other pulsed cryogenic load systems.